1. Field of the Invention
This invention relates to a substrate for a recording medium, used in a magnetic recording medium mounted in an external storage device for a computer and various other magnetic recording devices, as well as to a magnetic recording medium using such a substrate for a recording medium.
2. Description of the Related Art
Advances in recent years toward high-density recording of magnetic disks have been accompanied by a transition of magnetic recording methods from conventional in-plane recording methods (longitudinal recording methods) to perpendicular recording methods. Through development of perpendicular recording methods, recording densities have been improved tremendously, and whereas the recording densities of in-plane recording methods had been limited to 100 Gbits/square inch, at present recording densities in excess of 400 Gbits/square inch have been achieved. However, the limit for the first generation of simple perpendicular magnetic recording is 400 Gbits/square inch. This is because in order to raise the recording density, the bit size must be made smaller, but if the bit size is made smaller, thermal fluctuations result in bit degradation, i.e., random magnetization reversals readily occur. In order to avoid such bit degradation due to thermal fluctuations, satisfaction of equation (1) below is a necessary condition.
[E 1]
                                                        K              u                        ⁢            V                    kT                >        60                            (        1        )            
In equation (1), Ku is the uniaxial magnetic anisotropy constant, V is the volume per bit of the magnetic recording layer, k is the Boltzmann constant, and T is the absolute temperature. The left side of equation (1) is called the thermal stability index.
That is, when the bit size is made smaller, the volume V necessarily decreases. In order to overcome the instability of thermal fluctuation, the thermal stability index must be made to satisfy equation (1), despite the decrease in volume V. When the temperature during usage is constant, in order to increase the thermal stability index, the value of the uniaxial magnetic anisotropy constant Ku must be increased. Ku is a constant which depends on the magnetic material, and is given by the relation of equation (2).
[E 2]
                              H          c                =                                            2              ⁢                                                          ⁢                              K                u                                                    M              S                                -                                    M              S                        ⁡                          (                                                N                  z                                -                                  N                  y                                            )                                                          (        2        )            
In equation (2), Hc represents the coercivity, Ms is the saturation magnetization, and Nz and Ny represent the demagnetizing field coefficients in the z direction and y direction, respectively.
From equation (2), it is seen that the coercivity Hc is proportional to Ku. That is, if a material with a large Ku is selected in order to overcome the above-described thermal fluctuations, then the coercivity Hc, which represents the strength of the magnetic field reversing the magnetization, also becomes large, so that reversal of the magnetization by a magnetic head becomes difficult; in other words, a phenomenon occurs in which information writing becomes difficult. These problems of (1) “reduced volume accompanying higher densities”, (2) “long-term stability of recording resulting from thermal fluctuations”, and (3) “difficulty of recording due to high Hc” are interrelated in a complex manner, constituting a so-called “trilemma”, so that it has not been possible to discover a solution through an extension of conventional approaches.
Of late, methods have been proposed to escape from such a trilemma. One such effective method is the thermally assisted recording method (see Japanese Patent Application Laid-open No. 2006-12249 and Japanese Patent Application Laid-open No. 2003-45004).
In thermally assisted methods, the above-described trilemma state is addressed by resolving the problem of (3) “difficulty of recording due to high Hc” leaving the other two problems. Specifically, when writing data using a magnetic head onto a magnetic recording medium employing high-Hc material, by irradiating the magnetic recording medium with light for a short length of time, the Hc of the heated recording medium is lowered for a short period of time, so that writing is possible even using a weak magnetic field. Long-term stability, which is affected by thermal fluctuations, can be secured by again cooling to the reading temperature in a short enough time that bit degradation due to thermal fluctuations does not occur.
In this way, research and development of prototype thermally assisted methods as next-generation perpendicular recording methods have begun, and in theory, the possibility of recording densities exceeding 1 Tbits/square inch has been suggested (FUJITSU, Vol. 58, No. 1, pp. 85-89 (2007)). However, while thermally assisted methods in principle have great potential and have been regarded as promising candidates for next-generation perpendicular recording methods so that they have been the subject of detailed studies in preparation for commercialization, at the same time various difficulties have been discovered.
One difficulty is the substrate. At present, the substrates actually used as substrates for magnetic recording media are aluminum substrates and glass substrates. Aluminum substrates have an NiP plated layer of approximately 10 μm on the surface of the base aluminum material, and are used primarily in desktop computers and non-portable HDD recorders. Glass substrates include amorphous-material substrates and crystallized glass substrates, and are used in notebook computers and other portable equipment. In addition, although not yet commercialized, silicon single-crystal substrates have also been proposed in the past (see Japanese Patent Application Laid-open No. 4-143946 and Japanese Patent Application Laid-open No. 6-195707).
In thermally assisted methods, optical irradiation during writing by a magnetic head is performed to locally and instantaneously raise the temperature of the desired portion and, when writing ends, the optical irradiation ends simultaneously. It is desirable that rapid cooling to the usage temperature occurs. In order to obtain such behavior, during heating, a low thermal conductivity is a desirable characteristic of the substrate. On the other hand, during cooling, a high thermal conductivity is a characteristic sought for the substrate. That is, if during heating an attempt is made to raise the local temperature dramatically using a small amount of energy, then it is desirable that the temperature not be raised outside of the target area. To this end, it is desirable that the thermal conductivity be low. On the other hand, during cooling it is desirable that cooling to the usage temperature take place as quickly as possible, in order that the information written to the minute heated portion can persist with stability; to this end, a material with high thermal conductivity is necessary, in order that the substrate can play the role of a heat sink.
Thermal conductivity varies greatly with the material, and is 1.8 W/(m·K) for glass substrate, 5.0 W/(m·K) for NiP film, and 126 W/(m·K) for silicon substrate. Hence, the thermal conductivity of a NiP film and a glass substrate is extremely low compared with a metal, such as aluminum (the thermal conductivity of aluminum is 230 W/(m·K)), so that during heating in a thermally assisted method excellent performance is exhibited and this heating performance theoretically permits attaining recording densities exceeding 1 Tbits/square inch. However, because NiP film and glass substrate have low thermal conductivities, during cooling the intended performance of the thermally assisted method cannot be obtained. Further, if writing and reading are continued in succession, the temperature of the magnetic recording layer does not fall sufficiently and phenomena are observed in which written information is unstable.
One important characteristic sought from substrates for recording media is mechanical strength. In the prior art, NiP-plated aluminum substrates, glass substrates, and the like have been used as substrates for recording media. Aluminum substrates have elasticity and cannot easily be broken. Measures can be taken to secure mechanical strength when glass substrates are used which are prone to brittle fracture. Methods to improve the mechanical strength of glass substrates include (1) methods to crystallize the glass and (2) treatment methods for chemically tempering the glass, which both induce compressive stresses in the substrate surface to heighten mechanical strength.
Silicon substrates, like glass, are brittle, and in particular comprise a single crystal, so that cracks tend to occur along a cleavage plane. One type of mechanical strength of substrates for recording media of note is “annular bending strength” which simulates the mechanical strength when the media inner peripheral portion is clamped as the recording media is incorporated into a hard disk drive (HDD). In a HDD, because the media is clamped at the inner periphery, fracture proceeds from the inner-peripheral end face when excessive force is applied to the media. When stress is born by a brittle material, such as a glass substrate or silicon substrate, stress is concentrated at the tips of cracks existing in the surface. Thus, the extent of cracks formed in the coring process to open a hole in the substrate affects the annular bending strength and extremely weak areas may exist depending on the distribution of crack depth. In order to prevent such areas, after coring and end face chamfering, the inner and outer peripheral end faces are polished to remove cracks so as to improve the annular bending strength.
Many HDD applications of late have been for portable equipment such as notebook computers where it is a requirement that the HDD does not break if the equipment is dropped. Substrates for the recording media thus are required to have high drop impact strength in addition to the conventional annular bending strength. “Drop impact strength” is a measure of whether there is rupture of the substrate when an HDD, into which recording media is incorporated, is fixed to a drop impact tester and subjected to impact, normally with a peak acceleration of 1000 G for a duration of approximately 1 ms. When measuring the above-described “annular bending strength”, a force is gradually applied to the substrate inner periphery to investigate whether substrate fracture occurs so that this test is a quasi-static rupture test. When measuring “drop impact strength”, on the other hand, the drop impact test is a dynamic rupture test in which acceleration is applied over an interval of approximately 1 ms so that the substrate vibrates and such force is applied a plurality of times to the inner-periphery clamped portion. Hence, a substrate with high annular bending strength need not necessarily have high drop impact strength. For example, a silicon substrate with a nominal diameter of 2.5 inches has an annular bending strength of 280 N, which is high compared with the annular bending strength of 150 N for a glass substrate of nominal diameter 2.5 inches. However, in drop impact tests with an acceleration of 1000 G×1 ms, the probability of rupture for a glass substrate is zero, compared with a high 30% probability of rupture for a silicon substrate. Hence improvement of the drop impact strength remains a concern for silicon substrates.
As explained above, even in the case of thermally assisted methods, which in principle are promising next-general perpendicular recording methods, a number of concerns have come to light as a result of accumulated detailed research. One such concern is the thermal conduction of the substrate. Specifically, in a thermally assisted method, a minute region in which information is written by a magnetic head must be rapidly heated and, moreover, that minute region must be rapidly cooled so that it is desired that the substrate have the mutually contradictory characteristics of low thermal conductivity during heating and high thermal conductivity during cooling. Moreover, at the same time it is desired that the mechanical strength, i.e., the annular bending strength and the drop impact strength, be high. Further, when depositing the component layers of the magnetic recording media, an electrical conductivity sufficient to enable free application of a bias voltage is also desired.